1. Field of the Invention
The present invention is generally directed to an apparatus that separates particulate material, such as sand, or other mineral particles from fluids produced from a subterranean formation. The apparatus is particularly useful for replacing sand filters and other similar devices in oil and gas wells.
2. Background
Oil and gas wells are drilled from the earth's surface, the well borehole penetrating into a subterranean hydrocarbon producing formation. Such formations are typically made up of porous rock. That is, oil does not occur normally in the manner contemplated by the general public, that is, in pools of liquid. Instead, hydrocarbons, such as oil and gas are typically found in a rock strata that has porosity and permeability sufficient that entrapped hydrocarbon can flow through the formation. As the porosity of a rock formation increases, the quantity of hydrocarbons in proportion to the mass of the rock can also increase. Further, as the permeability of a rock formation increases, the more freely hydrocarbons can flow from the formation.
In the course of completing an oil and/or gas well, it is common practice to run a string of casing into the well bore and then to run the production tubing inside the casing. At the well site, the casing is perforated across one or more production zones to allow production fluids to enter the casing bore. During production of the formation fluid, mineral particles, often referred to as sand, may be swept into the flow path. The sand may erode production components, such as the downhole pump or sucker rod pump, the control valves on the surface, etc. in the flow path. In some completions, the well bore is uncased, and an open face is established across the oil or gas bearing zone. When substantial quantities of sand are carried along as oil and/or gas is removed from a formation, the sand can eventually plug the openings into the interior of tubing by which the hydrocarbon production is withdrawn to the earth's surface.
The hydraulic fracturing of the formations may also be another source of mineral particles in production fluids (i.e. oil and other hydrocarbons mixed with water). During hydraulic fracturing, propant or other fine mineral particles are injected into the openings and fractures created by the application of hydraulic pressure on the formation. The intended role of the injected propant or mineral particles is to keep the fractures open once the hydraulic pressure is released and production of oil begins. In some cases however, the backflow of oil, water and other production fluids wash the propant or mineral particles from the crack. As with sand particles previously discussed, the presence of propants or other mineral particles in the produced fluids results in the premature water of the down hole pumps and equipment.
For the above stated reasons and other reasons which should be known to one or ordinary kill in the art, it is important to prevent as much sand, propant or other mineral particles as is possible from entering into production tubing in a well and, for this purpose, filters, also known as sand filters or sand screens, have long been employed in the petroleum industry.
One or more sand screens may be installed in the flow path between the production tubing and the perforated casing (i.e. fully completed well) or the open well bore face (i.e. partially completed well). Often a rubber packer is customarily set above the sand screen to seal off the portion of the well bore where production fluids flow into the production tubing. The annulus around the screen may be packed with a relatively coarse sand or gravel which acts as a filter to reduce the amount of sand reaching the screen. A work string and service seal unit (SSU) is typically used to spot the gravel around the screen. During well completion, gravel may also be pumped and squeezed into the producing formation around the screen for filtering unconsolidated material out of the infilling well fluid. The gravel is pumped down the work string in a slurry of water or gel and is spotted directly under the packer or above the sand screen. The gravel also typically fills the annulus between the sand screen and the well casing. In well installations in which the screen is suspended in an uncased open hole, the gravel pack supports the surrounding unconsolidated formation.
Conventional sand screens typically employ a perforated mandrel which is surrounded by longitudinally extending spacer bars, rods or ribs and over which a continuous wire is wrapped in a carefully spaced spiral configuration to provide a predetermined longitudinal gap between the wire turns. For example see, U.S. Pat. No. 3,785,409; U.S. Pat. No. 3,958,634; and U.S. Pat. No. 3,908,256. The aperture between turns permits formation fluids to flow through the screen, while the closely spaced wire turns exclude free particulate material such as sand or gravel which may penetrate the gravel pack.
A problem which arises during initial production following the gravel packing operation is that fine sand may be carried through the gravel pack before the gravel pack bridge stabilizes. It is not unusual to produce a substantial amount of such fine sands before the gravel pack finally consolidates and yields clean production. During the early stages of producing the well after gravel packing, those fines tend to migrate through the gravel pack and screen and lodge within the inner annulus between the outer wire wrap and the perforated mandrel. In some instances, this can cause severe erosion of the screen and ultimate failure of the screen to reduce sand invasion. In other situations, the plugging materials may be carbonaceous, siliceous or organic solids which can completely plug the flow passages and terminate production shortly after completion. In deep wells, when the screen becomes plugged and the pressure in the production tubing is reduced, the formation pressure can collapse the screen and production tubing. Moreover, when a substantial amount of sand has been lost from the surrounding formation, the formation may collapse with resultant damage to the well casing or liner and consequent reduction or termination of production.
One attempt to overcome the foregoing problems is to interpose a prepack of gravel within the annulus between the inner mandrel and the outer wire screen. The prepacked gravel is sized appropriately to exclude the small mineral particles which accompany the formation fluid during production. Raw gravel, as well as epoxy resin coated gravel, have been used extensively in prepacked well screens. Some prepacked well screens are subject to retrieval problems due to their outer diameter being larger than that of a conventional well screen. In order to make prepacked well screens more easily retrievable, the inner mandrel is usually downsized, therefore imposing restrictions on both production and completion tool string bore sizing. Some prior art well screens have utilized an inner wire cloth or steel wire woven fabric filter media in order to achieve maximum annular placement and retention of prepacked filter materials. See, for example, U.S. Pat. No. 4,858,691 and U.S. Pat. No. 4,856,591. Such woven wire retainers do not provide free flow comparable to the conventional rib-channel design which is characteristic of resistance welded well screens. The wire mesh retainer which is wrapped directly onto the perforated mandrel only permits free flow to occur where it overlaps flow passages on the mandrel. Even in this instance, flow through the perforations is further restricted where the wire mesh retainer overlaps itself. The prior art sand screens which utilize fine wire woven retainers can result in plugging due to the fact that the openings in the wire mesh are typically considerably smaller than the flow openings in the outer screen member.
Additional information about sand filters and their application in oil or gas wells, see the following U.S. Pat. Nos. 1,367,406; 4,649,996; 1,992,718; 2,342,913; 2,985,241; 2,877,852; 2,858,894; 3,327,865; 4,064,938, 4,317,023; 4,388,968; 4,406,326; 4,428,431; 4,495,073; 4,494,603; 4,526,230; 4,649,996; 4,681,161; 4,811,790 and 4,821,800. Some filtration systems include the use of packing material, and for reference to these type of filters see U.S. Pat. Nos. 5,082,052; 5,115,864 and 5,232,048.
The sand filters described above can be considered deficient in several ways. In one way, many of the materials used for providing sand filtration do not possess the required ability to screen out small sand particles, that is, the screening materials let small diameter sand through that can ultimately cause blockage of flow paths or high pump wear as noted above. A second problem is that they have relatively small external surface areas so that sand builds up, blocking further flow. It should be apparent to one of ordinary skill in the art that longer filter life can be achieved in direct relationship to the external surface area of the filter. Unfortunately, many of the materials that have been used in previous type sand filters do not lend themselves to designing a sand filter in a way to increase the external surface area. A third problem is that many materials that are used for filtration are not capable of withstanding the harsh environment of a producing well. Corrosion of the wire filter components of sand filters is one of the principle cause of the failure of the sand filter to function properly. This is especially true in situations in which the oil being produced is a "sour" oil in which corrosive sulfur and/or nitrogen containing compounds are present. A fourth problem with some type filters is that they are expensive to manufacture and difficult to install. A fifth problem is that a high hydrostatic head is generated due to the filtering action of the sand filter. This results in lower overall production and higher cost for pumping the production fluids to the surface.
It should be apparent to one of ordinary skill in the art that there exists a continuing need in the petroleum industry for a simple, inexpensive device that effectively removes mineral particles from the production fluids prior to the production fluids being pumped to the surface.